The present invention relates to tensioning devices for timing chains. The tensioner of the present invention provides the advantages of a conventional hydraulic tensioner, but eliminates the hydraulic pressure system by use of spring loaded wedge-shaped blocks and friction. A wedge-shaped plunger pushes on two spring loaded wedge-shaped blocks to maintain the tension on a chain as it slackens.
Tensioners are used as a control device for a chain drive in an automobile engine timing system. The tension in the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. A tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system.
Most modern engines use hydraulic tensioners alone or with some type of mechanical rack to hold the position of the piston during start up or while the engine is off. A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a hollow piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and force of the spring in an outward direction.
As an engine speed increases, the torsional oscillations from the camshaft will cause the chain tension to increase. As the chain wears and stretches, the piston of the hydraulic tensioner protrudes outward to take up the excess chain length. The tension in the chain varies with the engine speed and the tensioner responds by adjusting the position of the piston to maintain chain tension. In a hydraulic tensioner, oil is allowed to flow into the piston as the piston moves outward. The tensioner relies on oil leakage to retract the piston as the speed lowers. A mechanical tensioner avoids the problems associated with maintaining hydraulic pressure in a fluid system.
In one conventional mechanical tensioner, shown in FIG. 4, a round plunger 200 is pushed up by a spring 202. The spring 202 contacts an end piece that is connected to the plunger 200 biasing the plunger 200 in a protruding direction. To prevent the plunger 200 from retracting once it has extended, balls 204 and 206 are spring loaded against the plunger shaft and are wedged between the plunger 200 and the angled surfaces 208 and 210 around the outside of the plunger. Specifically, the balls will stop the plunger because as the plunger retracts, the balls move into the more narrow space between the plunger and the angled surface. When there is no tension in the chain, then the balls will return to their original position because there is no pressure from the plunger. As a result, the plunger will also return to its original start-up position. This tensioner is used primarily for racing engines where camshaft timing is more important than wear on the timing drive components.
Another mechanical tensioning device is described in Kraft, U.S. Pat. No. 4,285,676. In Kraft, a housing is mounted in a fixed position on the vehicle engine adjacent the drive belt. A lever is pivotally mounted on a shaft, which is located within the housing and extends radially outwardly from the housing and is adapted to move in a belt tensioning direction. A first pair of cylindrical, torsional coil springs is mounted on the shaft, with a spring located on each side of the lever. A second pair of cylindrical, torsional coil springs is mounted on the shaft and surrounds the first spring pair. One end of each spring is attached to the housing with the other spring ends attached to the lever. The spring pairs are placed in torsion and bias the lever in the belt tensioning direction. An idler pulley is mounted on the end of the lever and is moved into tensioning engagement with the drive belt by the torsion springs. Alternatively, one of the vehicle accessories is mounted on the end of the lever for tensioning the drive belt engaged by the accessory pulley. The Kraft design avoids the use of a hydraulic system. However, Kraft does not use frictional forces, but rather a spring arrangement with three or more torsional springs.
Similarly, the present invention addresses the problems of wear and constant force on the chain. Instead of a hydraulic chamber, the piston is acted upon by a stacked belleville spring assembly, a return spring located on the outside of the housing, and the friction force between the piston and a pair of wedge-shaped blocks. In the present invention, a wedge-shaped plunger is biased by a return spring and a pair of spring loaded wedge-shaped blocks. This assembly has many advantages over a hydraulic tensioner or the prior art mechanical tensioners. For example, during start-up or hot idling, oil pressure is not present, so a hydraulic tensioner must draw oil in from the engine. If the engine has been off for a long time, the oil is no longer available and the piston will draw in air. The plunger will move in and out and not control the chain. This lack of control can cause start-up noise or the chain may slip a tooth.
In contrast, in the present invention, instead of hydraulic pressure, the plunger angle and friction coefficient control the resistance force against inward movement of the tensioner. The tensioner force pushing the plunger in will not exceed the total force of the return spring, wedge springs, and static friction, so the force on the chain will not exceed the endurance limit. Thus, the tensioner will respond effectively in all conditions. Additionally, the tensioner can be mounted anywhere in the engine, and there is no potential fluid leak down path.